Bacteriorhodopsin (BR) is a retinal protein molecule found in the photosynthetic system of a salt-marsh bacterium called Halobacterium salinarium. The BR molecules are located in the cell membrane, forming a 2D protein-lipid array, commonly called the purple membrane. The use of photochromic proteins like bacteriorhodopsin (BR) for optical data storage has been considered promising. BR-based optical films have been worked on for the past two decades, but they do not have the required properties to make them commercially viable for data storage applications. One of the problems with the BR-based films is that BR forms 0.2-1 μm sized protein-lipid patches. If BR is extracted from these patches to form a monomeric protein, it becomes unstable and is inactivated in a few days. The problem with using these BR patches in optical films is that the patches are approximately the same size as the wavelength of the light used to interface the film. This results in significant light scattering during read and write cycles, thereby increasing noise and degrading the performance of the film. Additionally, the BR patches tend to stick to each other, which result in uneven distribution of the BR protein in the film, and further degrade the performance of BR-based optical films.
Another problem of BR is that it is expensive to produce in large quantity. BR has to be expressed in its natural organism H. salinarum for it to be fully functional (Dunn, et al., J Biol Chem, 262: 9246-9254 (1987); Hohenfeld, et al., FEBS Lett, 442: 198-202 (1999)). H. salinarum grows very slowly, gives a low cell density and requires the presence of large amounts of salt in the growth medium. The low productivity of H. salinarum and the need for expensive custom-made fermentation and recovery equipment that can tolerate the high salt growth medium result in high cost of BR production.
Proteorhodopsins (PRs) are distantly related to bacteriorhodopsin (22-24% sequence identity). Proteorhodopsins are integral membrane proteins; they are isolated from uncultivated marine eubacteria and function as light-driven proton pumps. Upon absorption of light by the all-trans-retinal co-factor, proteorhodopsin goes through a photocycle with a number of intermediates. It is believed that upon excitation of the proteorhodopsin molecule by light stimulation, a proteorhodopsin/retinal complex is excited to an unstable intermediate energy state. Proteorhodopsin progresses through a series of unstable energy states that can vary in terms of energy plateaus or intermediates, e.g., an “M-like state” or “M-state”, a “K-like state” or “K-state”, an “N-like state” or “N-state”, or an “O-like state” or “O-state”. Subsequently, the complex reverts to a more stable basal state concomitant with transportation of a proton.
Proteorhodopsin and bacteriorhodopsin are different families of proteins. These proteins have some shared characteristics, but also have clearly different properties. Proteorhodopsins are more advantageous to use in some technical applications than bacteriorhodopsins because of the ease of expressing and producing proteorhodopsins. Proteorhodopsin can be functionally expressed in E. coli, a bacterial host capable of rapid high-level protein expression. Thus, production of proteorhodopsin is more economic and efficient than production of bacteriorhodopsin.
Béjà, et al. (Science 289:1902-6, 2000) disclose the cloning of a proteorhodopsin gene from an uncultivated member of the marine γ-proteobacteria (i.e., the “SAR86” group). The proteorhodopsin was functionally expressed in E. coli and bound all-trans-retinal to form an active light-driven proton pump.
Béjà, et al. (Nature 411:786-9, 2001) disclose the cloning of over twenty variant proteorhodopsin genes from various sources. The proteorhodopsin variants appear to belong to an extensive family of globally distributed proteorhodopsin variants that maximally absorb light at different wavelengths.
Dioumaev, et al. (Biochemistry, 42: 6582-6587 (2003)) disclose using proteorhodopsin-containing membrane fragments encased in polyacrylamide gel for flash photolysis and measurements of absorption changes in the visible range.
Optical data storage has the potential to revolutionize the computer industry, since optical data storage provides both a very high storage capacity and rapid reading and writing of data. Additionally, optical signal processing could be used in a highly parallel fashion for pattern recognition, which is difficult to do with the current computing technologies. A functional optical material with low light scattering is required for these applications to succeed.
Documents like banknotes, checks, identity cards etc. often incorporate security features to make them difficult to copy or counterfeit. Most of these are based on either using special paper with security features like watermarks incorporated during paper manufacturing, or printing hairline patterns that are difficult to copy. However, such features are permanently visible and do not meet security requirements.
There are needs for optical information carriers that can be produced efficiently and economically and have low background noise. Such optical information carriers are effective as optical data storage material or fraud-proof optical data carriers.